You guys are just awesome. The stuff you have posted has been a huge help. Not only for filling in the gaps, but giving me enough tips to point me in the right direction to go and research further 
Board arrived on Friday. Read 'Getting Started'over the weekend, and thumbed through 'Practical Arduino' as well. Got s/w and stuff set up and did some basic stuff Monday night to get a feel for things (blinky LED etc).
Stuck a photo interrupter on the breadboard last night, and managed to promptly fry that one by mis-reading the data sheet (thought it was bottom up, not top down view... DOH!). Anyway, the PI's I got have a schmitt trigger in them, which made life easy.
Got a sketch up and running using an interrupt on pin3, and a tight output loop for the serial, as well as some extra timing such as printing a new timestamp ~ every 1 sec. Just a single PI, but my total laptime count was only about 20 millis out from the total millis count, after about 10 mins and 300 test lap events, and no dropped serial data.
Its all just really getting used to things and unit-testing the various concepts and 'areas' on the projects at this point. I'll post my code tonight if you feel like a laugh.
Rob